Section D at 26 through 30 of Form 4473 contains the firearm’s Manufacturer and/or Importer, the Model, the Serial Number, Type, and Caliber or Gauge of every firearm involved.
After the transaction, FFL guy must make the Form 4473 and any other supporting documents a part of his permanent records which must be retained for at least twenty years.
Whether or not a NICS check was required makes not a bit of difference, a Form 4473 must be completed anyways.
Then if by stroke of the pen, Federal agents swoop down on and demand every FFL guy give up all their records, the ATF will have the information needed to register every firearm involved in every transaction where a Form 4473 was filed going back over the last 20 years.